Fullscreen not working

Playing Youtube videos in fullscreen does not work. All it does is open up a new standalone window, although large, but not fullscreen. Vivaldi 1.2.470.11 (Utviklerversjon) dev (64-bit) Versjon ff228a374881cbd34de7a4c15c12681679de2fe0 Blink 537.36 (@ff228a374881cbd34de7a4c15c12681679de2fe0) JavaScript V8 5.0.71.34